In general, it is important to cool semiconductor chips, such as processor chip, to maintain reliable operation and prevent thermal damage to electronic components. It is more problematic and difficult to implement effective mechanisms for cooling 3D chip stacks as compared to singular chips, and the ability to efficiently cool a chip stack can limit the height and total power of a chip stack. Common cooling techniques for chip stacks include the use of high-performance water cooling systems on a backside of the chip stack, but this technique is not adequate for a stack structure with many chips or a chip stack having a high-power chip on a bottom of the stack. While a water-cooled thermal interposer can be used at the bottom of the chip stack, this structure is difficult to integrate and requires isolation of thru silicon vias (TSVs) from the liquid coolant that is used. If a dielectric fluid is used as the coolant, isolation of the TSVs is not required. With single phase cooling, the performance of dielectric fluids is inferior to water.
Other cooling techniques include two-phase cooling in which a liquid coolant having a relatively low boiling point is used (e.g., liquid which evaporates at an operating temperature of the chips being used). With two-phase cooling in closed channels, the heated liquid evaporates to create an annular flow wherein a thin liquid film (evaporation layer) is present on the surfaces being cooled, and heated evaporated coolant flows through confined channels outlet ports. With this cooling process, the latent heat of the liquid coolant is typically much larger than the specific heat of the fluid times the typical temperature increase of the liquid coolant. As such, as compared to pure liquid cooling techniques, two-phase cooling can provide greater cooling ability using a much lower volume of coolant fluid, lower coolant mass flow rates and lower operating pressure. Advantages of two-phase cooling include the ability to select the boiling temperature of the coolant or use an expansion valve for refrigeration.
However, it is very difficult and problematic to control two-phase flow through microchannels or other manifold structures that are typically used for two-phase cooling systems. Indeed, a two-phase flow tends to be unstable and can vary in the same or different regions of a microchannel or manifold structure. Moreover, the increased volume of the vapor phase results in high vapor velocity causing substantial pressure drops and potentially disrupts the thin evaporation layer on the channel walls, leading to local dry out (i.e., dewetting of surfaces to be cooled).